WASHINGTON — Oil that gushed out of BP’s failed Gulf of Mexico well in 2010 may have left a 1,235-square-mile “footprint” on the deep-ocean floor, according to new research published Monday.
The study, published in the Proceedings of the National Academy of Science, suggests that “significant quantities” of crude from the destroyed Macondo well may have been transported near deep-sea corals southwest of the site.
It also helps answer a question of scientific — if not legal — dispute since the Macondo well blew out on April 20, 2010: What was the fate of all that oil?
An estimated 4.9 million barrels are believed to have escaped the well, with much of it burned off or broken up with chemical dispersants. Some of the crude washed ashore. An estimated 2 million barrels of oil is believed to have gone unrecovered.
To find out where it went, a team of researchers led by University of California Santa Barbara microbial geochemist David Valentine and including representatives from UC Irvine and the Woods Hole Oceanographic Institution, analyzed data from more than 3,000 samples collected at 534 locations around the ruptured BP well.
Their work focused on identifying hopane, a tracer hydrocarbon that indicated the presence of crude. While it was found throughout the northern Gulf of Mexico, the researchers discovered it was most concentrated in a thin layer on the sea floor within 25 miles of the Macondo well.
“Our findings suggest that these deposits come from Macondo oil that was first suspended in the deep ocean and then settled to the sea floor without ever reaching the ocean surface,” Valentine said.
Light, freshly released oil normally is generally not expected to sink, and even dispersed oil is more likely to remain suspended in the water column.
Figure from research published in the Proceedings of National Academy of Sciences in October 2014 showing hydrocarbon contamination at the seafloor near the failed Macondo well. Dots indicate affected coral communities. Lower right inset depicts the molecular structure of hopane, a tracer hydrocarbon used as a proxy identifying crude. (Image courtesy of G. Burch Fisher)
Valentine described the footprint on the sea floor as a “shadow of the tiny oil droplets that were initially trapped” higher up, in the water above. “Some combination of chemistry, biology and physics ultimately caused those droplets to rain down another 1,000 feet to rest on the sea floor,” he added.
The 1,235-square-mile area he identified is almost twice the size of the city of Houston, which spans 620 square miles.
BP suggested that the researchers inaccurately attributed the tracer hopane to Macondo crude.
“The authors failed to identify the source of the oil, leading them to grossly overstate the amount of residual Macondo oil on the sea floor and the geographic area in which it is found,” said BP spokesman Jason Ryan. “Instead of using rigorous chemical fingerprinting to identify the oil, the authors used a single compound that is also found in every natural oil seep in the Gulf of Mexico, causing them to find false positives all over the sea floor.”
Although the PNAS paper documents scattered, patchy deposits, the “mapping technique connects the sample locations as if the oiling were continuous between the sampling points,” Ryan added. “This dramatically overestimates the impacted area.”
Complicating any analysis of spilled crude are natural seeps of oil. According to the National Research Council, natural seeps release an estimated 560,000 to 1.4 million barrels of oil into the Gulf of Mexico each year.
But Valentine argues that the pattern of contamination “is fully consistent with the Deepwater Horizon event” and “not with natural seeps.” For instance, much higher concentrations of the tracer hopane were found at the top, superficial layers of sediment, suggesting they were dumped there recently — not part of a slow, natural accumulation over time. The higher concentrations close to the well were also markedly different than the “background” amount documented further from the site.
Valentine’s footprint calculations suggests that once-suspended oily particles were sinking through the water near deep-sea corals about seven miles away from the well.
Previous, unrelated research published in 2012 by the National Academy of Sciences linked the Gulf oil spill with damage to the corals, which were discovered months after the Deepwater Horizon blast.
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